A vehicle that includes an automatic transmission may have a torque converter positioned between the vehicle's engine and the automatic transmission. The torque converter provides a viscous fluidic coupling between the engine and the automatic transmission. By coupling the engine to the transmission via fluid, it is possible to rotate the engine without rotating the transmission output at lower engine speeds. Further, the transmission input can rotate at nearly engine speed while the engine operates at higher speeds. Accordingly, the efficiency of torque transfer through the torque converter improves at higher engine speeds. In addition, engine torque supplied to the automatic transmission may be multiplied by a factor between 1 and 3 when the engine operates between low and high speeds.
A torque converter is often designed with a fixed K factor. The K factor is a constant that allows a torque converter to be characterized independent of the engine coupled to the torque converter, and it is expressed as RPM at stall speed divided by square root of torque at stall speed. The torque converter K factor is related to the torque converter's turbine, impeller, and stator designs. A torque converter may be designed with a high K factor to increase torque multiplication at lower engine speeds. However, the high K factor torque converter may not provide a desired vehicle response at other operating conditions where less torque multiplication is desired. On the other hand, a torque converter may be designed with a low K factor to transfer engine torque more efficiently at lower engine speeds. However, a vehicle including a torque converter with a low K factor may not launch as quickly as is desired during some conditions.
The inventors herein have recognized the above-mentioned limitations and have developed a method for operating a vehicle powertrain, comprising: increasing a K factor of a torque converter in response to a torque provided via an engine being less than a desired torque. For example, if a vehicle wheel torque is less than desired, the torque converter K factor can be increased so that additional wheel torque may be observed.
By adjusting the torque converter K factor in response to a torque provided by an engine being less than a desired torque, it may be possible to provide more consistent vehicle launches during varying vehicle operating conditions. In one example, the torque converter K factor is increased during vehicle launch so that torque multiplication by the torque converter increases so as to increase wheel torque. Additionally, increasing the torque converter K factor may allow engine speed to increase since less engine torque is needed to accelerate the vehicle. And, increasing the engine speed may allow the engine to reach an operation condition where more torque may be provided by the engine. Thus, vehicle launch may be improved during conditions that would otherwise degrade vehicle launch.
The present description may provide several advantages. In particular, the approach may improve vehicle launch. Further, the approach may improve vehicle drivability during cold conditions. Further still, the approach may provide for a more consistent wheel torque over a wider range of vehicle operating conditions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.